A multi-celled extremophile that can survive in unprecedented temperatures has been found in pools of heated water within Lassen Volcanic National Park in California.
The discovery has potential repercussions in the search for alien life, widening the range of environments that could potentially support complex organisms.
Organisms that can survive in extreme environments are classed as extremophiles. When that extreme environment is related to heat, then the extremophiles are called "thermophiles." The new thermophile found in volcanic waters has been called Incendiamoeba cascadensis, nicknamed the "fire amoeba." It was found able to self-replicate via cell division at a temperature of 63 degrees Celsius (145 degrees Fahrenheit) and was still fully active, albeit no longer reproducing, at 64 degrees C (147 degrees F). Even at 66 degrees C (150.8 degrees F), the amoeba was still partially active, and while it shut down at 70 degrees C (158 degrees F), it was able to recover. However, 80 degrees C (176 degrees F) killed the flame amoeba.
Single-celled organisms have been found thriving at much greater temperatures. Methanopyrus kandleri is a single-celled organism that survives and reproduces in temperatures as high as 122 degrees C (252 degrees F). However, single-celled prokaryotes such as M. kandleri lack a cell nucleus and cell membrane, both of which can break down when subjected to significant heat, and the lack of these cellular components helps M. kandleri survive at temperatures well past the boiling point of water.
Complex, multicellular life — classed as "eukaryotes" in the tree of life — is more vulnerable to high temperatures. Unlike prokaryotes, eukaryotes do have cells with a nucleus and a membrane. The previous record for the highest temperature in which multicellular organisms were able to reproduce was 60 degrees C (140 degrees F) for some species of fungi and red algae, and researchers had grown pessimistic about the ability of eukaryotes to thrive at higher temperatures.
"In part, studies on eukaryotes may have been limited because of assumptions about membrane stability," Beryl Rappaport, a graduate student at Syracuse University and the lead scientist behind the research, said in a statement. "We are hoping that the discovery of I. cascadensis encourages others to keep searching for high-temperature eukaryotes."
The key to understanding the fire amoeba lies in its gene sequence. Rappaport's team were able to identify genes that stabilize and protect DNA at 63 degrees C, and also genes that allow the fire amoeba to sense its environment and the temperature around it. In some cases, gene activity even increased at high temperature, such as I. cascadensis' ability to fold proteins to serve various biological functions.
"We were able to uncover many strategies that could help I. cascadensis survive at high temperatures, and some of these strategies could be used by thermophiles across all life," said Rappaport. "For instance, some proteins in I. cascadensis have a high positive surface charge that could help them remain stable. These protein charges are similar to those found in thermophilic bacteria and archaea [single-celled microbes including prokaryotes]."
Intriguingly, the fire amoeba's DNA contains sequences of genes similar to DNA fragments found in geothermal samples as far and wide as New Zealand and the United States' Yellowstone National Park. The implication is that I. cascadensis, or its cousins, could exist all around the world and are just waiting to be discovered.
Similar thermophiles might exist beyond Earth, too. No other environments in the solar system share the temperate environment of Earth — there's either too much heat or too much cold, too much radiation or not enough light, too much acidity or toxic materials. And beyond our solar system we have yet to find a planet that has an environment like Earth. However, some extraterrestrial environments could potentially support what we would consider to be extremophiles — for example, on Mars or the Jupiter ocean moon Europa. The existence of I. cascadensis therefore broadens the range of temperatures in which eukaryotic life — and humans are also considered eukaryotes — can survive on other worlds.
"It could certainly be possible for complex life like I. cascadensis to survive on another planet, but Earth is the only planet we currently know of to have all the requirements for I. cascadensis to be happy," cautioned Rappaport. "It's not just about temperature. An environment also needs the right acidity, oxygen levels, pressure, water and food. I. cascadensis could not survive on its own. It needs other life to be supported as well."
So a thermophile would not be able to survive on airless, barren Mercury despite the high temperature. Thermophiles could, however, find a home on worlds slightly closer to their star than the traditional habitable zone or on highly volcanic worlds, so long as water is also present, which is not the case for Jupiter's volcanic moon Io.
The new study was published on Sept. 22 in the journal Cell.
